Effects of an increasing filter feeder stock on larval abundance in the Oosterschelde estuary (SW Netherlands)

Predation by adult bivalves on bivalve larvae has been suggested to reduce larval abundance in areas with high bivalve filter-feeder biomass. Although the occurrence of larviphagy is well-studied in the laboratory, its effects in the field have scarcely been studied. We studied larviphagy at different spatial scales in the Oosterschelde estuary. On the scale of individuals, we confirmed that larviphagy occurs in Crassostrea gigas and Mytilus edulis in the Oosterschelde estuary, by examining stomach contents of adult bivalves. On a local scale, we studied effects of larviphagy by a Pacific oyster (C. gigas) bed on presence of larvae in the overlying water column by sampling larvae with fixed plankton nets. Abundance of blue mussel (M. edulis) larvae was significantly reduced by the oyster. Abundance of C. gigas larvae did not seem to be reduced by the oyster bed, but spawning by the adult oysters during the sampling period may have affected the results. On estuary-scale, the effect of larviphagy on larval abundance of C. gigas and M. edulis was studied using existing monitoring data over 6 years for M. edulis and 13 years for C. gigas. Numbers of M. edulis larvae showed no significant trend over the 6 years studied. Abundance of C. gigas larvae declined with an increasing filter feeder stock (that was mainly caused by an increase in C. gigas stock). This decline may be due to direct effects of larviphagy or indirect effects such as lowered food levels, and was not compensated by an increased larval production. All results combined, complemented with a theoretical estimate of the effect of larviphagy on estuary-scale, strongly suggest that larviphagy is major source of mortality for bivalve larvae in the Oosterschelde estuary.